How can I backup all the photos and videos in my iPhone 13 with “back up with iCloud?

"iCloud Photos > Manage Storage > 26.9 GB" -- that is the size of your iCloud Photos on iCloud. iCloud stores your photos at their full size.

"Settings > General > iPhone Storage > Photos it shows size of Photos (Documents & Data) as 962.7 MB" - that is the size of the photos on your iPhone; the photos on your phone can be optimized to smaller, space-saving versions.

Photos on the iPhone can take up less storage space if you have the Optimize iPhone Storage option turned on. See the following from Set up and use iCloud Photos - Apple Support

iCloud Photos keeps all your photos and videos in their original, high-resolution version. To save storage space on your device, you can turn on Optimize Storage.

If you turn on Optimize Storage, iCloud Photos automatically manages the size of your library on your device. Your original photos and videos are stored in iCloud and space-saving versions are kept on your device. Your library is optimized only when you need space, starting with the photos and videos you access least. You can download the original photos and videos over Wi-Fi or cellular when you need them.

If you turn on Download Originals, iCloud Photos keeps your original, full-resolution photos and videos in iCloud and on your device.

Also be aware that iCloud Backup and iCloud Photos are not the same thing.

If you have iCloud Photos turned on you can use an internet browser and view the photos at www.icloud.com and you also have the option to Optimize the photos on your phone. If iCloud Photos is on then photos will not be included in iCloud Backup.

If iCloud Photos is off, then photos can be included in iCloud Backup. You cannot view the contents of iCloud Backup; you can only use it to do a full restore or to setup a new device. Also there is no option to optimize the size of the photos on the phone.